A number of techniques have been developed recently to meet the demands for rapid and accurate detection of infectious agents, such as viruses, bacteria and fungi, and detection of normal and abnormal genes. Such techniques, which generally involve the amplification and detection (and subsequent measurement) of minute amounts of target nucleic acids (either DNA or RNA) in a test sample, include inter alia the polymerase chain reaction (PCR) (Saiki, et al., Science 230:1350, 1985; Saiki et al., Science 239:487, 1988; PCR Technology, Henry A. Erlich, ed., Stockton Press, 1989; Patterson et al., Science 260:976, 1993), ligase chain reaction (LCR) (Barany, Proc. Natl. Acad. Sci. USA 88:189, 1991), strand displacement amplification (SDA) (Walker et al., Nucl. Acids Res. 20:1691, 1992), Qβ replicase amplification (QβRA) (Wu et al., Proc. Natl. Acad. Sci. USA 89:11769, 1992; Lomeli et al., Clin. Chem. 35:1826, 1989) and self-sustained replication (3SR) (Guatelli et al., Proc. Natl. Acad. Sci. USA 87:1874-1878, 1990). While all of these techniques are powerful tools for the detection and identification of minute amounts of a target nucleic acid in a sample, they all suffer from various problems, which have prevented their general applicability in the clinical laboratory setting for use in routine diagnostic techniques.
One of the most difficult problems is preparation of the target nucleic acid prior to carrying out its amplification and detection. This process is time and labor intensive and, thus, generally unsuitable for a clinical setting, where rapid and accurate results are required. Another problem, especially for PCR and SDA, is that conditions for amplifying the target nucleic acid for subsequent detection and optional quantitation vary with each test, i.e., there are no constant conditions favoring test standardization. This latter problem is especially critical for the quantitation of a target nucleic acid by competitive PCR and for the simultaneous detection of multiple target nucleic acids.
Circumvention of the aforementioned problems would allow for development of rapid standardized assays, utilizing the various techniques mentioned above, that would be particularly useful in performing epidemiologic investigations, as well as in the clinical laboratory setting for detecting pathogenic microorganisms and viruses in a patient sample. Such microorganisms cause infectious diseases that represent a major threat to human health. The development of standardized and automated analytical techniques and kits therefor, based on rapid and sensitive identification of target nucleic acids specific for an infectious disease agent would provide advantages over techniques involving immunologic or culture detection of bacteria and viruses.
Reagents may be designed to be specific for a particular organism or for a range of related organisms. These reagents could be utilized to directly assay microbial genes conferring resistance to various antibiotics and virulence factors resulting in disease. Development of rapid standardized analytical techniques will aid in the selection of the proper treatment.
In some cases, assays having a moderate degree of sensitivity (but high specificity) may suffice, e.g., in initial screening tests. In other cases, great sensitivity (as well as specificity) is required, e.g., the detection of the HIV genome in infected blood may require finding the virus nucleic acid sequences present in a sample of one part per 10 to 100,000 human genome equivalents (Harper et al., Proc. Nat'l. Acad. Sci., USA 83:772, 1986).
Blood contaminants, including inter alia, HIV, HTLV-I, hepatitis B and hepatitis C, represent a serious threat to transfusion patients and the development of routine diagnostic tests involving the nucleic acids of these agents for the rapid and sensitive detection of such agents would be of great benefit in the clinical diagnostic agree laboratory. For example, the HIV genome can be detected in a blood sample using PCR techniques, either as an RNA molecule representing the free viral particle or as a DNA molecule representing the integrated provirus (Ou et al, Science 239:295, 1988; Murakawa et al., DNA 7:287, 1988).
In addition, epidemiologic investigations using classical culturing techniques have indicated that disseminated Mycobacterium avium-intracellulaire (MAI) infection is a complication of late-stage Acquired Immunodeficiency Syndrome (AIDS) in children and adults. The precise extent of the problem is not clear, however, since current cultural methods for detecting mycobacteria are cumbersome, slow and of questionable sensitivity. Thus, it would be desirable and highly beneficial to devise a rapid, sensitive and specific technique for MAI detection in order to provide a definitive picture of the involvement in HIV-infected and other immunosuppressed individuals. Such studies must involve molecular biological methodologies, based on detection of a target nucleic acid, which have routinely been shown to be more sensitive than standard culture systems (Boddinghaus et al., J. Clin. Med. 28:1751, 1990).
Other applications for such techniques include detection and characterization of single gene genetic disorders in individuals and in populations (see, e.g., Landergren et al., Science 241: 1077, 1988 which discloses a ligation technique for detecting single gene defects, including point mutations). Such techniques should be capable of clearly distinguishing single nucleotide differences (point mutations) that can result in disease (e.g., sickle cell anemia) as well as deleted or duplicated genetic sequences (e.g., thalassemia).
The methods referred to above are relatively complex procedures that, as noted, suffer from drawbacks making them difficult to use in the clinical diagnostic laboratory for routine diagnosis and epidemiological studies of infectious diseases and genetic abnormalities. All of the methods described involve amplification of the target nucleic acid to be detected. The extensive time and labor required for target nucleic acid preparation, as well as variability in amplification templates (e.g., the specific target nucleic acid whose detection is being measured) and conditions, render such procedures unsuitable for standardization and automation required in a clinical laboratory setting.
The present invention is directed to the development of rapid, sensitive assays useful for the detection and monitoring of pathogenic organisms, as well as the detection of abnormal genes in an individual. Moreover, the methodology of the present invention can be readily standardized and automated for use in the clinical laboratory setting.